1. Technical Field
This invention relates to the field of cable television systems generally and, more particularly, to a method and apparatus for interdicting cable television channels of a cable television system such that the audio portion of a television channel to be interdicted is effectively jammed as well as the video portion.
2. Description of the Related Arts
So-called interdiction type cable television systems are known, for example, from U.S. Pat. Nos. 4,450,481; 4,651,204 and 4,912,760. These patents are representative only and generally describe a method and apparatus for permitting television channels, including audio and video portions, to be transmitted in the clear (not scrambled) from a central location known as a cable television headend and down a cable to an interdiction unit, typically mounted on a pole or on the side of a cable television subscriber's residence. At the interdiction unit, at least one voltage controlled oscillator hops from cable television channel to cable television channel to jam the television channel in order to deny the subscriber access to the interdicted channels. The remaining channels, including basic service and those the subscriber has paid for, are permitted to pass the interdiction unit in the clear into the subscriber's residence to be viewed on their television set. Because cable television channels are transmitted in the clear, there are no residual artifacts from scrambling and descrambling processes known in the scrambling arts.
In prior art interdiction systems, the jamming carrier is typically placed between the video and audio carriers and transmitted at such an amplitude level to at least deny reception of the video portion of the signal and, to a limited extent, at least, degrade the audio portion. The audio signal jamming is not particularly effective in such systems. A compromise must be reached between jamming the video (more important) and audio portions of the television channel to be interdicted.
The single jamming carrier for jamming both audio and video is permitted to dwell on a particular television channel for a period of time known as the dwell or on time and then hops at a hopping rate to jam other channels that the subscriber is not authorized to receive. Thus, oscillators may be shared among many channels to be jammed. In a simple one oscillator system, as the effectiveness of jamming is increased, the fewer the channels the oscillator has to jam since dwell time inherently increases with decreasing numbers of channels to jam.
On the other hand, these references generally teach that a delicate balance must be maintained between jamming the carrier signal level, the frequency of the jamming carrier and the signal level of an adjacent channel, either immediately above or below the channel to be jammed, so that the adjacent television channel is not adversely impacted by the jamming signal. This phenomenon which is undesirable is known as adjacent channel interference.
Jamming effectiveness is improved the longer the jammer is permitted to dwell on a particular channel. Parikh et al., U.S. Pat. No. 5,278,908, teach principles of addressable interdiction systems and varying the jamming effectiveness and dwell times for different channels depending on their video content sensitivity, for example, violent or sexual content.
None of these systems, including the prior art systems referenced above, have concentrated any attention to audio portion jamming. Audio jamming is performed in these systems at the same time as video jamming and by the same jamming oscillator. The same oscillator that is used for video portion jamming is typically used for audio portion jamming. Consequently, it is still possible, depending on the jamming effectiveness for audio jamming determined by the dwell time, hopping rate, jamming signal location, frequency and amplitude of the jamming carrier, to hear and intelligibly distinguish speech or other audible sounds that may be disturbing to children. Consequently, there remains a need in the art to provide a system, in the first instance, which effectively jams the audible portion of a television signal and, in the second instance, does so without causing adjacent channel interference. Moreover, when the subscriber turns to a jammed channel, the subscriber should not be able to intelligibly distinguish speech or sounds but should be able to listen to an interdicted audio signal which may sound like noise that is not an extremely objectionable sound and does not offend their sensitivity to vulgar or indecent language.
Since conventional interdicting systems are incapable of high level jamming of the audio portion of a television program signal, there remains a need in the art to provide such a system.